Atmospheric Pressure Chemical Vapor Deposition

ABSTRACT

A process for coating a substrate at atmospheric pressure comprises the steps of vaporizing a controlled mass of semiconductor material at substantially atmospheric pressure within a heated inert gas stream, to create a fluid mixture having a temperature above the condensation temperature of the semiconductor material, directing the fluid mixture at substantially atmospheric pressure onto the substrate having a temperature below the condensation temperature of the semiconductor material, and depositing a layer of the semiconductor material onto a surface of the substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of provisional patent applicationSer. No. 60/602,405, filed Aug. 18, 2004.

FIELD OF THE INVENTION

The present invention relates generally to the deposition of a vaporizedchemical material on a substrate, and more particularly to a process fordepositing a vaporized chemical material on a substrate at atmosphericpressure.

BACKGROUND OF THE INVENTION

Chemical vapor deposition processes such as pyrolytic processes andhydrolytic processes are well known in art of coating substrates. Thephysical characteristics of the coating reactants utilized in suchprocesses may be liquid, vapor, liquids or solids dispersed in gaseousmixtures, aerosols, or vaporized or vaporous coating reactants dispersedin gaseous mixtures.

In the process of deposition of a vaporized chemical compound on a glasssubstrate in the production of photovoltaic devices, the vaporizedchemical compound is typically deposited in a vacuum atmosphere. Thesystems for carrying out such process have typically included a housinghaving an enclosed deposition chamber formed of a lower portion and anupper portion with a horizontal junction with each other. A sealassembly is interposed at the junction between the lower and upperhousings. A conveyor means is provided to transport glass sheetsubstrates through the chamber. A chemical vapor distributor is locatedwithin the deposition chamber to provide a coating on the glasssubstrate as the substrate passes through the chamber.

The system includes a vacuum source for drawing a vacuum within thedeposition chamber. The deposition chamber typically includes elongateheaters for heating the glass sheets as they are conveyed through thesystem. The glass sheets pass into the deposition chamber from avacuum-heating furnace to the vacuum deposition chamber that ismaintained at a similar vacuum and temperature setting as the heatingfurnace. Powdered cadmium sulfide and powdered cadmium telluride are fedinto the vaporization deposition chamber. The films are then depositedonto the previously coated and heated glass substrates sequentially. Thecoated substrates are next transferred through a load lock and thenceinto a cooling chamber wherein cooling is effected by compressednitrogen and finally conveyed to atmosphere pressure though an exit loadlock to an air cooling section for reduction to ambient temperature. Thecadmium telluride thin-film material requires a follow-on processingstep to re-crystallize its polycrystalline structure so that effectivephotovoltaic devices can be made from the film stack. Typically thisstep is accomplished by applying a solution of cadmium chloride to thecadmium telluride surface of the cooled coated glass and re-heating theglass to a temperature of about 390° C. to 420° C. for a period of about15 to 20 minutes. Care must be taken to slowly heat and cool the glassto avoid breakage during this treatment which extends the overallprocess time of the required step.

Since it is well recognized that renewable energy sources are becomingincreasingly more important, it is deemed that commercial production ofphotovoltaic devices for the generation of electrical energy isimportant in satisfying the renewable energy needs. The utilization ofthin-film coatings of semiconductor materials on glass substrates isconsidered to be a viable mechanism in the field of photovoltaic-basedelectrical energy generation systems.

It has been found that thin-film coating systems, based upon the abovereferred to technology, are capable of depositing thin film of cadmiumsulfide/cadmium telluride photovoltaic material onto commerciallyavailable soda-lime glass substrates in a vacuum. The photovoltaicmaterials are subsequently treated to re-crystallize the cadmiumtelluride surface making the film stack ready for further processinginto photovoltaic devices. While the above-described system is capableof producing photovoltaic panels suitable for the production ofelectrical energy, it would be desirable to reduce the cost for suchproduction to render the system commercially viable.

SUMMARY OF THE INVENTION

It is an object of the present invention to produce a photovoltaic panelby depositing thin-films of semiconductor materials from chemical vaporson a substrate at atmospheric pressure.

Another object of the present invention is to produce a photovoltaicpanel by vaporizing cadmium sulfide and cadmium telluride and depositingthe same on the surface of a heated substrate to form a first thin filmof cadmium sulfide and a second thin film of cadmium telluride atatmospheric pressure.

Another object of the present invention is to produce a photovoltaicpanel by rapid high temperature re-crystallization of the thin-filmcadmium telluride polycrystalline material to yield high efficiencyphotovoltaic devices.

It has surprisingly been found that the above objects may be achieved bya process for coating a substrate at atmospheric pressure, comprisingthe steps of: (1) providing sources of semiconductor materials such ascadmium sulfide or cadmium telluride; (2) heating and vaporizing thesemiconductor materials at substantially atmospheric pressure, andmaintaining the vaporized materials at temperatures above theirtemperatures of condensation; and (3) sequentially depositing thevaporized materials on a heated surface of a substrate such as glass atsubstantially atmospheric pressure, to form a laminar structure.Optionally, while the laminar structure is still substantially at thetemperature of deposition and at substantially atmospheric pressure, acadmium telluride layer may be treated with a reactive gas to effect are-crystallization of the cadmium telluride. Subsequent processing ofthe laminar film stack may be accomplished to produce active thin-filmphotovoltaic devices.

BRIEF DESCRIPTION OF THE DRAWING

The above as well as other objects and advantages of the invention willbecome readily apparent to those skilled in the art from reading thefollowing detailed description of a preferred embodiment of theinvention in the light of the accompanying drawing, in which:

The drawing shows in schematic form the inventive steps of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawing, there is schematically illustrated the stepsof the process for coating a surface of a substrate with a film ofcadmium telluride at atmospheric pressure.

Individually metered masses of semiconductor material, preferablycadmium sulfide (CdS) or cadmium telluride (CdTe) in powdered form, areintroduced into a zone which is continuously purged by a stream of inertgas, preferably nitrogen, flowing between an inlet and an outlet atapproximately atmospheric pressure. The powder is carried from theinlet, by the inert gas flowing at a controlled rate, into a vaporizerconsisting of a heated packed bed in which the powder is vaporized as itpasses through the interstitial voids of the packed bed's media. Theoutlet of the heated packed bed is caused to communicate with theinterior of a heated zone to distribute the vaporized material to thesubstrate. Alternative powder vaporization methods through which themetered powder mass and carrier inert gas are heated may be employed forgenerating the vaporized material fluid stream. The alternative methodsmay include, but are not necessarily limited to, heated fluidized bedsin which the carrier inert gas is heated and the powder is vaporized,thermal “flash” vaporizers that heat the carrier inert gas and vaporizethe powder, and atmospheric pressure thermal spray units that heat thecarrier inert gas and vaporize the powder.

The fluid, preferably including cadmium sulfide or cadmium telluridepowder and the carrier inert gas, is a high temperature flowing mixturecomprising the carrier inert gas and vaporized material at a temperatureabove its condensation temperature. The temperature of the fluid mixtureis typically in a range of from about 800° C. to about 1,100° C. Theheated fluid mixture is then directed into an apparatus for producing alaminar flow of constant velocity toward the surface of a substrate atsubstantially atmospheric pressure. The substrate is typically asoda-lime glass, preferably having a low-E coating that is transparentand electrically conductive. An example of such glass is produced byPilkington Glass Co. and is designated as TEC-15. The surface of thesubstrate is maintained at a temperature of from about 585° C. to about650° C.

The apparatus for producing the desired laminar flow of fluid mixturecomprises of a series of individual passageways adapted to cause aseries of velocity changes in the transient fluid as the fluid flowsthrough the passageways. The apparatus is maintained above thevaporization temperature of the cadmium sulfide or cadmium telluride, toprevent condensation of the material within the passageways. Such fluidflow evenly distributes the fluid mixture to an elongate outlet nozzle,and enables uniform laminar flow at constant mass flow distribution toflow tangentially of and supplied to the surface of the substrate. Theabove action causes the molecules of the fluid mixture to be evenlydistributed throughout the length of the elongate outlet nozzle, andcauses the molecules to travel from the outlet nozzle in a generallyparallel path and at a constant velocity, producing a laminar flow ofconstant velocity and mass distribution directed toward the substrate.

The velocity of the fluid mixture exiting the outlet nozzle may beregulated by controlling the mass flow rate at which the fluid mixtureis introduced at the inlet.

In order to control the thin-film deposition rate of the vaporizedmaterial within the fluid emitted from the apparatus being applied tothe substrate, the mass flow rate of the fluid mixture and the velocityof the substrate are controlled while controlling the temperature of thesubstrate below the vaporized material's condensation point. As theheated fluid mixture impinges onto the cooler substrate, it cools to atemperature below the condensation temperature of the vaporizedmaterial. The material condenses from the fluid mixture, in apolycrystalline form, onto the moving substrate as a continuousthin-film layer. A fluid extraction device is disposed upstream anddownstream of the outlet nozzle, to enable the controlled withdrawal ofthe non-film generating constituents of the fluid mixture directed tothe surfaces of the substrate.

While there may be a number of different systems for evenly distributingthe vaporized cadmium sulfide or cadmium telluride on the surface of thetransient glass substrate, it is contemplated that the apparatusillustrated and described in U.S. Pat. No. 4,509,526 to Hofer et al. mayprovide satisfactory results.

The deposition of any number of consecutive layers of cadmium sulfideand/or cadmium telluride by the apparatus described above, to prepare alaminar structure, is contemplated by the present invention.

Subsequent to the deposition of a cadmium telluride polycrystallinethin-film, a re-crystallization step would be required to allow theproduction of photovoltaic devices from the laminar thin-film stack. Ithas been found that this step can be achieved in less than one minute bysubjecting the hot cadmium telluride film to a hot gaseous atmosphere ofdilute hydrogen chloride in nitrogen at substantially one atmosphere ofpressure. The ability to control the re-crystallization of the cadmiumtelluride while maintaining the temperature of the substrate eliminatescool-down and re-heating of the substrate/film-stack assembly during there-crystallization step. The use of a “dry” re-crystallization stepeliminates the use of a toxic cadmium chloride solution and itsapplication apparatus. Typically, a glass substrate exiting the in-linere-crystallization process would have a temperature from about 620° C.to about 630° C. This temperature range allows the glass to be thermallytempered by cool quenching gas flows as the substrate/film-stack exitsthe processing line.

The above-described process relates to a method for producing athin-film cadmium sulfide/cadmium telluride photovoltaic material on thesurface of a soda-lime glass substrate, to provide large areaphotovoltaic panels. However, it must be understood that the concept ofatmospheric vapor deposition can be extended to include other thin-filmmaterials that are normally deposited in a vacuum.

Thin-film photovoltaic materials that could be considered are CIGS(copper-indium-gallium-diselenide), CdS/CIS-alloy (cadmiumsulfide/copper-indium-selenium alloy), amorphous silicon or thin-filmpolycrystalline silicon, and Zn (O, S, OH)_(x)/CIGS (zinc oxide sulfidehydroxide/copper-indium-gallium-diselenide).

Other thin-film materials that can be considered for application toglass substrates are optical coatings such as multi-layer stacks usedfor very low emissivity films and anti-reflection films. Other valueadded features such as improved durability films, self-cleaning films,photo-optic, and electro-optic films could be developed using theinventive atmospheric pressure deposition concept.

The process of atmospheric pressure deposition of thin-film materialscould be applied to a variety of substrate materials for enhancement oftheir surface properties. Substrates that could be considered includepolymeric materials, ceramics, metals, wood, and others.

1. A process for coating a substrate at atmospheric pressure, comprisingthe steps of: vaporizing a controlled mass of semiconductor material atsubstantially atmospheric pressure within a heated inert gas stream, tocreate a fluid mixture having a temperature above the condensationtemperature of the semiconductor material; directing the fluid mixtureat substantially atmospheric pressure onto the substrate having atemperature below the condensation temperature of the semiconductormaterial; and depositing a layer of the semiconductor material onto asurface of the substrate.
 2. The process according to claim 1, whereinthe semiconductor material comprises cadmium sulfide or cadmiumtelluride.
 3. The process according to claim 1, wherein the inert gas isnitrogen.
 4. The process according to claim 1, wherein the temperatureof the fluid mixture ranges from about 800 degrees C. to about 1,100degrees C.
 5. The process according to claim 1, wherein the substratecomprises glass.
 6. The process according to claim 5, wherein the glassbears a transparent, electrically conductive low-E coating.
 7. Theprocess according to claim 1, wherein the substrate has a temperatureranging from about 585 degrees C. to about 650 degrees C.
 8. The processaccording to claim 1, wherein the steps of vaporizing, directing, anddepositing are repeated at least once, to deposit at least oneadditional layer of semiconductor material on the substrate.
 9. Aprocess for coating a substrate at atmospheric pressure, comprising thesteps of: vaporizing a controlled mass of semiconductor material,comprising cadmium sulfide or cadmium telluride, at substantiallyatmospheric pressure within a heated nitrogen gas stream, to create afluid mixture having a temperature ranging from about 800 degrees C. toabout 1,100 degrees C.; directing the fluid mixture at substantiallyatmospheric pressure onto a glass substrate bearing a transparent,electrically conductive low-E coating and having a temperature rangingfrom about 585 degrees C. to about 650 degrees C.; and depositing alayer of the semiconductor material onto a surface of the substrate. 10.The process according to claim 9, wherein the steps of vaporizing,directing, and depositing are repeated at least once, to deposit atleast one additional layer of semiconductor material onto the substrate.11. A process for coating a substrate at atmospheric pressure,comprising the steps of: vaporizing a controlled mass of semiconductormaterial, comprising cadmium sulfide or cadmium telluride, atsubstantially atmospheric pressure within a heated nitrogen gas stream,to create a fluid mixture having a temperature ranging from about 800degrees C. to about 1,100 degrees C.; directing the fluid mixture atsubstantially atmospheric pressure onto a glass substrate bearing atransparent, electrically conductive low-E coating and having atemperature ranging from about 585 degrees C. to about 650 degrees C.;depositing a layer of the semiconductor material onto a surface of thesubstrate; and repeating the steps of vaporizing, directing, anddepositing at least once, to deposit at least one additional layer ofsemiconductor material onto the substrate.